Navigation systems or head-up displays (hereinafter, also referred to as HUDs) are used as a technique of presenting information to drivers of motor vehicles, aircrafts, etc. HUD is a system that projects an image emitted from an image projection unit, such as a liquid crystal display body (hereinafter, also referred to as an LCD), to, for example, a windshield of a motor vehicle.
FIG. 1 is a schematic diagram showing a HUD. As indicated by an optical path 5, light emitted from an image display units 2 reflects off the reflective mirror 3, reflects off a windshield 4, and thereafter reaches an observer 1. The observer 1 is viewing an image projected on the windshield 4, but the image appears as if it is at an image position 6 which is farther than the windshield 4. With such a technique, a driver can obtain various information while focusing ahead of the windshield 4 with almost no movement of his/her line of sight, and thus it is safer than conventional car navigation systems which require movement of the line of sight.
HUD projects display information such that the display information overlaps a view that is actually seen through the windshield 4, and thus it is desirable to display a bright and clear image without obstructing a view. To this end, it is necessary to have both transmittance sufficient for a view ahead to be viewable and reflectance sufficient for a reflection image of the HUD to be viewable. However, transmittance and reflectance are normally in a trade-off relationship, and there is a drawback that if one of them is increased, the other becomes low.
Particularly, for an automotive windshield, it is difficult to apply a member having high reflectance due to a legal limit that limits a visible light transmittance in a direction vertical to glass to be greater than or equal to 70%. Concerning such a drawback, Patent Documents 1 to 4 disclose techniques in which a combiner having light polarizing units, polarized light modulating units, and reflective polarizing units is provided in a laminated glass of the windshield, and the polarized light modulating units is controlled depending on the brightness of the surroundings to control the transmittance and the reflectance of the combiner.
Patent Document 5 discloses a technique in which a cholesteric liquid crystal layer is used as units of reflecting light and disposed in an automotive windshield.